function f = biharmonic_u(x,y,caseNum,varargin)
switch caseNum
    case 1
        f = 4*exp(x+y);
    case 2
        f = 0*x;
    case 3
        f = 0*x;
    case 4  % stokes 1
        f = 48;  
    case 5  % stokes 2
        f = 100*sin(x+3*y);
    case 6  % stokes 3
        f = 25*exp(2*x+y);
    case 7  % stokes 4
        f = 96./(1+x+y).^5;
    otherwise
        f = 0 * x;
end